1. Field of the Invention
The present invention relates to a niobate-based piezoelectric material composition and a method for producing the same; more specifically, the invention relates to an alkali metal-containing niobate-based piezoelectric material composition for use in the knocking sensor of automobile engine and the like.
2. Description of Related Art
Piezoelectric materials have been used conventionally for the knocking sensor of an automobile engine. Piezoelectric materials are broadly divided as piezoelectric materials containing lead-based substances and piezoelectric materials not containing lead-based substance. These are selected and used, depending on the use. As known, lead is a hazardous substance. Hence, piezoelectric materials containing lead-based substances essentially contain the hazardous substances. Therefore, caution should be paid during the handling and use thereof. Thus, piezoelectric materials of excellent characteristic properties but with no content of hazardous substances such as lead-based substances are demanded.
The piezoelectric materials with no content of lead-based substances are known, which are produced for example by adding various auxiliary components to niobate-based materials (ANbO3); e.g., A: alkali metal, thereby elevating the sinterability of the materials to highly densify the materials and thereby raise the stability thereof over time. When the sinterability is enhanced in such a manner, the components and phases of the materials can be densified to final high densities. Hence, the resulting piezoelectric constants electromechanical coupling factors and mechanical quality factors thereof are relatively high. Additionally, piezoelectric materials are of great safety performance because of no content of hazardous substance, and with great piezoelectric properties and high stability.
As auxiliary components to be added to such niobate-based materials, use is made of various materials. For example, Japanese Patent Laid-open No. Sho 50(1975)-47193 discloses a composition composed of a composition formula KNbO3-NaNbO3-LiNbO3 with addition of manganese oxide; additionally, Japanese Patent Publication No. Sho 60 (1985) -52098 discloses a composition composed of a general formula Na1xe2x88x92xLixNbO3 (0.02xe2x89xa6xc3x97xe2x89xa60.30) with addition of aluminum oxide within a range of 0.02 to 2.0% by weight and iron oxide within a range of 0.001 to 0.019% by weight. These are both intended to promote the sinterability of a material and thereby highly raise the relative density, thereby improving the electromechanical coupling factor to recover a material with large mechanical strength.
However, the niobate-based piezoelectric materials are so poor in terms of material sinterability and temperature properties including temperature stability at high temperature and stability over time, that the material properties are deteriorated, depending on the time elapsed in day, although the niobate-based piezoelectric materials have excellent properties at first. Thus, the niobate-based piezoelectric materials are prepared by molding above the re-crystallization temperature and pressing (by the hot press method). Compared with a so-called sintering method comprising molding a material at atmospheric pressure and then sintering the material, a sintered material at a high density can thereby be prepared at low temperature. However, the method is disadvantageous in that the production cost by the method is high and the method can never produce a larger sintered material. Thus, the material prepared by the hot press method has improved sinterability and is thus highly densified owing to the action of the auxiliary component therein. But the temperature stability and stability over time thereof are not satisfactory; and additionally, the production cost is disadvantageously high.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide an alkali metal-containing niobate-based piezoelectric material composition by which a substitutional solid solubilization agent so as to improve the temperature property and stability over time of niobate-piezoelectric materials not containing lead-based materials can be found.
Another object of the present invention is to provide a niobate-based piezoelectric material composition which can be prepared by the existing sintering method at atmospheric pressure, as a preparation method thereof; and to provide a method for producing a niobate-based piezoelectric material composition which utilize the existing sintering method at atmospheric pressure.
Additional objects and advantages of the invention will set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided an alkali metal-containing niobate-based piezoelectric material composition comprising: a solid solution represented by a composition formula (ANbO3) (A: alkali metal); and at least one additive selected from Cu, Li and Ta.
Preferably, the solid solution may be represented by a composition formula (K1xe2x88x92xNaxNbO3) (wherein x=0 to 0.8). In this case, the at least one additive may preferably be Cu having an amount of 0.001 to 5 mol %.
Alternatively, the solid solution may preferably be represented by a composition formula Lix(K1xe2x88x92yNay)1xe2x88x92x(Nb1xe2x88x92zTaz)O3 (wherein x=0.001 to 0.2, y=0 to 0.8, z=0 to 0.4). In this case, the at least one additive may preferably be Cu, Li and Ta, each of them having an amount of not more than 5 mol %; or alternatively the at least one additive may preferably be Cu, Li and Ta, the Cu being in an amount of 0.001 to 5 mol %.
In such a manner, a material with good properties can be recovered because Cu acts as a sintering auxiliary agent for solid solution so that the relative density can be improved. Furthermore because Cu acts on the domain in the solid solution with an effect on the stabilization of the domain, the stability over time can be improved. Thus, a piezoelectric material with such a good property as described above can be provided. When the amount of Cu to be added then is below 0.001 mol %, Cu cannot function as a sintering auxiliary agent; while Cu is added above 5.0 mol %, the resulting properties are poor, disadvantageously. Thus, the optimum amount occupies 0.001 to 5.0 mol %.
Furthermore, the reason why Li and Ta are added is as following. Li acts as a sintering auxiliary agent because Li is sintered in the liquid phase during sintering; Ta acts on the ferroelectric domain in the solid solution to fix the ferroelectric domain and stabilize the dielectric loss. Through these actions, a piezoelectric material with great properties can be provided. As to the amount of Li then, an amount below 0.1 mol % or above 20 mol % is disadvantageous in that the resulting piezoelectric properties are deteriorated. Accordingly, the optimum amount occupies 0.1 to 20 mol %.
According to the present invention, the action of the additive (e.g., an additive powder containing Cu) as a sintering auxiliary agent can highly densify the material. Still furthermore, the action of the additive (e.g., an additive powder containing Cu) for substitutional solid solubilization can prevent the reduction of dielectric loss, so that excellent piezoelectric properties with good stability over time can be recovered. Because the material cannot contain any lead-based material, a material with great safety and high piezoelectric performance can be produced.
According to the present invention, the Li functions as a sintering auxiliary agent for promoting the high densification of the material and the Ta acts on the ferroelectric domain of a solid solution to reduce the dielectric loss and improve the dielectric constant, with the resultant recovery of a material with great stability over time and temperature stability. Because the resulting material does not contain any lead-based material either, the material can be used for a wide variety of use.
This invention also provides a method for producing an alkali metal-containing niobate-based piezoelectric material composition comprising: adding an additive powder containing at least one element selected from Cu, Li and Ta to a mixture powder represented by a composition formula ANbO3 (A: alkali metal), then blending these powders together; molding said mixture powders and sintering the same; and giving piezoelectricity to the resulting sintered-substance in a process of a polarization treatment.
Preferably, the sintering process may be an atmospheric pressure sintering method or a mechanically pressed sintering method. In this case, the sintering process may preferably be carried out with a heating method selected from a group consisting of electric furnace heating, microwave heating, high frequency induction heating, and infrared heating.
Furthermore, the additive powder may preferably be 0.001 to 5 mol % of Cu, and the mixture powder may preferably be K1xe2x88x92xNaxNbO3 (x=0 to 0.8). Alternatively, the mixture powder may preferably be Lix(K1xe2x88x92yNay)1xe2x88x92x(Nb1xe2x88x92zTaz)O3 (wherein x=0.001 to 0.2, y=0 to 0.8, z=0 to 0.4), and the additive powder may also preferably be 0.001 to 5 mol % of Cu.